Brett, I think I remember you set up for a 424 with the pipe on the 61 Brett version? Ro-Jet claims the 67 is a stroked out 61 torque monster intended for muffler people. I don't know if my thinking is right but is the steady 4 stroke with muffler set up easier to operate?
No, my setup runs in a 4-stroke more-or-less all the time, although, when it is off a bit, it runs across the break. That's the key feature that makes it better, when running across the break, the power difference is not excessive. That's the same as when Ted and David were running the 46VF with the Randy AAC, the breaks where extremely gentle, almost non-events, if everything else was right. That was always where we had issue with the PA61, I could never get it to run without excessive power change at the break without also losing power everywhere else. My best PA setup was also a constant 4-stroke as well. Someone at the 2001 or 2002 NATs came up and congratulated me on my great run, and how I had the best-running Saito on the field!
Just so everyone is clear, the fact that these engines run in a constant 4 (or a constant 2) doesn't mean they are "constant-speed" engines. There is plenty of response in the maneuvers whether it changes state or not. The pipe response controls it, not the misfiring (or not) you get in a 4-stroke. I like the 40VF because you can easily adjust it to have a bit too much boost in the corners, or a bit too little, with dead-nuts repeatability with simple pipe length changes, and once you set it, it stays that way. My current RO-Jett system is the best I have had, because if set correctly, it has just the right response, if I get a little fast, the response goes 'flat" to minimize the acceleration and make the maneuvers easier, and if it is set a bit too slow, it picks up enough boost to make up for the too-slow run. It also starts breaking in the maneuvers when set too fast, but that has almost no effect on the power.
For the constant 4-stroke run, I adjust the prop diameter, venturi, and nitro to get it to run in a 4 throughout the flight. More diameter causes more feedback to the engine, and cause it to want to break more. A smaller venturi or less nitro causes you to run generally leaner, which puts it closer to the break to begin with. Less nitro reduces the power change at the break, more increases it, so you trade off how much nitro to either avoid the break entirely (more nitro) or let it break and moderate the power change (less nitro). On both the PA and Jett, I run *far less* diameter than the engine could otherwise handle. Both could easily be set up to run 14" props if you wanted to, but if you try to compete with Paul/David/Ted on a regular basis, you won't like your 14" prop (any more than Windy or others did...). I run a 12.5-3.75 3-blade with no undercamber, and at one point (2000 NATs, where I finished tied for 3rd), I flew with David's "break-in" prop - a 12-3.75 undercambered 3-blade that had been ground down on bad takeoffs to around 11.5". That was before we discovered that we should be running 15% nitro, so it took a pretty small prop to keep it in a 4 all the time.
For piped engines, I think it is much easier to just adjust the overall capability up and down to control the operating point, rather than attempt to deal with the combination of pipe tuning regulation effects and phase changes at the same time. By far, the biggest issue I see is that people have their piped systems set to give FAR TOO MUCH boost/brake, and many of them have no idea how much it is hurting their progress, they don't know that you shouldn't have to panic when you turn the left top corner on the square 8 and the engine breaks into a screeching 2-stroke and accelerates you at the ground. I have flown airplanes that would go about halfway around a corner, break, and nearly pull the handle out of my hand, and the owner is thrilled by how much power they have. The judges are usually less-than-thrilled at the side effects.
I haven't run my 61s (PA or Jett) on a muffler. Ted ran his 61 that way for a while and it ran fine, but he said it gave up a fair bit of performance that way. If nothing else, you are running it generally slower, meaning more pitch (~4.5-5" as I recall for Ted) and that alone means less performance in most aspects.
I have only seen 67s with mufflers, and they also all flew well and ran nicely. I will admit that I haven't spent a lot of time with modern engines on mufflers, and certainly not enough time to see if the fiddle-factor is significantly improved over the good old days. The endless fiddling to get it to break at the right time and the right amount as the conditions changed was one of the big reasons everybody dropped them like hot potatoes as soon as something less labor- and knowledge-intensive came along. That was also of course when we were using engines like the ST46 and ST60 which sometimes acted like random number generators as far as what they did, or sometimes just went off the rails for no particular reason. And, you had to keep them running in perfect form because you couldn't afford to run them a bit "off" as the performance was too marginal to permit that.
A modern super-engine like the 67 would very likely remove much of the randomness and certainly the power margin would be better, and maybe that would reduce the fiddling necessary enough to make it worth doing. But one of the reasons you can put amazingly powerful engines like the PA75 in a teeny little ST46-sized airplane like the Thundergazer and come up with a world-beater is because you can control the power with the pipe and other adjustments.
MANY people, not understanding this, figured, "gee, I can get a 91, why do I need a pipe?" I figure you need a pipe even more with a 91 than with a 40 - because otherwise, you have to try to dump the 90% of the possible power, instead of maybe 25% with a 40. That's why Windy couldn't get the Jett 88 or 90, or whatever it was, to work - 1% too much and it was way too fast, 1% too little and it was way too slow. He switched back to a 76 in the middle of the NATs one year because of that. In the good old days, you absorbed power my making the airplane bigger, but we are already about as big as you can practically go. For a 91, you might need a 1000-square-inch airplane to give yourself some tolerance, but that's far too large to be practical in a 70-foot circle.
That's more-or-less a core dump of my knowledge of the topic, most of us switched to piped engines because of the improved performance and reduced fiddling, and at this point, we have figured out a number of reliable systems and settings (which is what took the knowledge and evaluation skills). But I expect that unless you are trying to compete with others at a high level, the mere fact that any of these engines are of amazing high quality compared to what we had 30 years ago in the 4-2 break era will reduce the randomness enough to reduce the endless diddling required before, and provide very good performance.
If you got in a time machine with two PA40SE's, set the controls to 1977, and handed them to someone like Ted Fancher, we would have retired the Walker Trophy at this point because he would have won every contest for the next 15 years (instead of just most of them...). That's because, compared to something like the ST46 or other engines of the time, even the least capable of these engines would have been an astonishing breakthrough. Heck, might have been the same if it was LA46s. Bascially anything you can get these days is better than the best stuff available back then, and will work better. So it kind of depends on what standards you are trying to achieve.
Brett
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